1. Field of the Invention
The present invention relates to a device for measuring the relative angular position of two bodies with respect to a point, and to an articular prosthesis provided therewith.
2. Description of the Related Art
As is known, active articular prostheses have been developed, which, when applied to a patient, enable a much more effective recovery of the functionality of the injured or missing limb than do traditional passive prostheses. In particular, active articular prostheses for the lower limbs (knee and ankle) tend to facilitate movement of the patient during deambulation.
Active articular prostheses comprise a pair of artificial skeletal members (for example, prostheses of tibia and femur), hinged to one another, so as to form an joint, and are provided with a control unit, an angular-position sensor, and an actuator, which is able to supply a torque between the artificial skeletal members of the joint. The angular-position sensor detects the relative angular position of the skeletal members, and the control unit, on the basis of the information supplied by the angular-position sensor, operates the actuator so as to control the movement of flexo-extension of the joint, especially during deambulation.
In known articular prostheses, angular-position sensors of a resistive type (angular potentiometers) or of an inductive type are normally used. Angular potentiometers are used for obtaining voltage dividers with variable voltage-division ratio. More precisely, an angular potentiometer is provided with a resistive element and a moving slider, in sliding electrical contact with the resistive element. The resistive element, fixed to one of the skeletal members, is thus divided into two resistive portions, and the division ratio between the two resistive portions depends upon the position of the slider, which is fixed to the other skeletal member. The information on the angular position of the skeletal members is hence supplied by the value of the voltage-division ratio. Angular sensors of an inductive type are based upon detection of the current flowing in a first winding on account of the variations in a magnetic field generated by a second winding, which is angularly movable with respect to the first winding. In particular, variations in the relative angular position of the two windings (each fixed to a respective skeletal member) modify the magnetic flux concatenated by the first winding, which is thus subject to an induced electromotive force and is traversed by a current.
Both of the types of sensors described, however, suffer from serious drawbacks that limit the performance and possibility of use thereof. In particular, angular potentiometers are readily subject to failure, have considerable encumbrance and usually require an extremely accurate assembly, in so far as even minimal misalignments are critical for their operation. In addition, angular potentiometers show problems of mechanical wear, on account of the sliding contacts. In the case of use for prostheses of the knee or ankle, these problems of wear are aggravated by the intensive use of the sensor. Angular sensors of an inductive type are more robust, have higher linearity, and do not present problems of mechanical wear. However, the encumbrance of inductive sensors is considerable, on account both of the electronic control circuits and of the shaft necessary for connection of one of the windings to the respective skeletal member. Inductive sensors are hence far from suitable for being miniaturized and integrated in a prosthesis.